In general, a surface acoustic wave("SAW") filter employing a piezo-electric material, to be used as a filter in high frequency band of more than 100 MHz, should have a high propagation velocity and electromechanical coupling coefficient, and the cost of piezo-electric material should be sufficiently low. However, the conventional piezo-electric material such as LiNbO.sub.3, has been proven to be less satisfactory in the senses that it is relatively expensive and is a single crystal structure with large volume. Aluminum nitride("A1N") film, an alternative piezo-electric material currently used, can be easily and inexpensively fabricated according to the sputtering methods conventional in the art. A1N film has been reported to have a SAW propagation velocity of above 5,000 m/s(see: H. Okano et al., Jpn. J. Appl. Phys., 33:2957(1994)). However, since the A1N film has been also known to have a low electromechanical coupling coefficient of below 0.8%(see: K. Tsubouchi and N. Mikoshiba, IEEE Trans. Sonics Ultrason., SU-32:634(1985)), there has been a continuous need for the piezo-electric A1N film having a high electromechanical coupling coefficient value.
On the other hand, Wang et al. suggested that, if 25% of nitrogen is replaced with hydrogen gas on deposition of A1N film, the rough surface becomes have a smoothness of about 0.3 nm, and the adhesiveness is improved due to the decrease of stress inside of the film(see: X.-D. Wang et al., Thin Solid Films, 251:121(1994); X.-D. Wang et al., Langmuir, 8:1347(1992)). Despite the improvement of the smoothness and the adhesiveness of A1N:H film containing hydrogen, there is still a problem of blistering phenonmenon that the chemical nature of the film is changed by the vapor of the atmosphere and peeled off.
Accordingly, there are strong reasons for exploring and developing alternative means for fabricating a surface acoustic wave filter having a high SAW propagation velocity and electromechanical coupling coefficient, while overcoming the blistering problem of the A1N:H film.